The invention relates to a sensor membrane of an optical sensor for detection of O.sub.2, H.sub.2 O.sub.2, SO.sub.2 or halogenated hydrocarbons in a sample with an indicator substance that is homogeneously immobilized in the polymer matrix of the sensor membrane and is at least indirectly, in contact with the sample, changing at least one of its optical properties upon a change of the parameter to be measured, this indicator substance containing an inorganic salt of a transition metal complex with .alpha.-diimine Iigands.